MICROGRAVITY STUDY OF THE DERBENDIKHAN DAM, NORTHEASTERN IRAQ

A microgravity investigation was conducted along the axis of the Darbandikhan rockfill dam, 230km NE of Baghdad, Iraq. The dam is constructed within the Western Zagros Fold-Thrust Belt which is a tectonically active region and the boundary between the Arabian and Asian plates. The goal of the survey was to evaluate the subsurface damages that could threaten the stability of the dam due to the effects of a 7.3 magnitude earthquake with the Zagros Fold-Thrust belt whicch caused cracking and landslides. The survey, which consists of 324 microgravity station distributed on four profiles, was performed using a high sensitivity CG-5 gravimeter. The data were processed into complete Bouguer gravity anomalies using high-precision elevation data. Residual gravity anomalies were obtained by removing by applying a 2m upward continuation filter. Several negative and positive anomalies were evident in the residual anomalies map which reflects the subsurface heterogeneity in density. Power spectrum analysis revealed six average depth slices of gravity sources: 0.95m, 1.75m, 5.5m, 12.5m, 21.0 and 55.0m. The results revealed that no recognizable indication regarding the evolution of subsurface channels after the earthquake. Furthermore, presented two prominent anomalies appear close to the right bank at roughly 21m in depth; the first is a positive anomaly approximately 8m long, and the second is a negative anomaly approximately 12m long. These positive and negative anomalies suggest high and low- density zones coincide with different degrees of compressional stresses caused by the earthquake. Further detailed about the subsurface was determined from 2D forward modeling of the residual gravity field. This modeling might interpret the subsurface high (compact) or low (less compact) density zones. The results can be used as a guide for a drilling program to examine the sources of the anomalies.